The present invention relates generally to the field of optical waveguide fibers, and more particularly to optical waveguide preforms and methods of making optical waveguide preforms, from which low water peak optical waveguide fibers are formed.
A significant goal of the telecommunications industry is to transmit greater amounts of information over longer distances, in shorter periods of time. Over time there has also typically been an increase in the usage of telecommunication systems, by users and by system resources. This has resulted in demands for increased bandwidth in the media used to carry this information over long distances, in particular for optical waveguide fibers which are contained in telecommunication cables.
Bandwidth in optical waveguide fibers is dependent on a number of factors, such as the attenuation of the fiber at the transmission wavelength. Impurities present in the light guiding region of the fiber can increase the attenuation of the fiber, due to absorption of the transmitted light. Of particular importance is the attenuation caused by the hydroxyl radical (OH), which can be bonded to the fiber structure during the manufacturing process. The presence of OH bonds in the light guiding region of the fiber can cause an attenuation increase, with a peak in a window around 1380 nm, also generally referred to as the water peak. The 1380 nm window is defined as the range of wavelengths between about 1330 nm to about 1470 nm; with the peak attenuation effect typically around 1383 nm.
In the past, telecommunications systems avoided using the water peak region, partly due to the lack of optical waveguide fiber with low water peaks. In recent times, however, fiber manufacturers have been producing low water peak fibers, by various methods, which has coincided with the development of telecommunication systems which increasingly use all the wavelengths between about 1300 nm and 1650 nm. In order for telecommunication systems to fully utilize this wavelength range, removal of the water peak from the optical waveguide fiber is essential.
There are three main methods of optical waveguide preform manufacture in common use. The three techniques have similar methods of vapor generation and oxidation, but differ in the geometry of the substrate on which the oxide soot is deposited;
(i) Deposition in Tube Methods
These methods comprise techniques known as MCVD (Modified Chemical Vapor Deposition) and PCVD (Plasma Chemical Vapor Deposition). In these techniques, a vapor stream is introduced to the end of a high-purity quartz tube, and the oxides are deposited on the inner surface of the tube.
(ii) VAD (Vapor Axial Deposition)
In this technique, the deposition takes place on a vertically mounted rotating mandrel and the preform is “grown” axially in the vertical plane, from a short stub into a longer, cylindrical preform.
(iii) OVD (Outside Vapor Deposition)
In this technique, silica-based soot is deposited on a rotating target rod. The rod builds up to form a cylindrical soot preform, which can be sintered and dried to form a glass preform.
Although the methods and embodiments of the present invention are generally applicable to all of the above techniques, it is particularly applicable to optical waveguide preforms manufactured using the OVD process.